In orthodontic surgical procedures, orthodontic brackets commonly are attached directly to teeth by adhesive bonding The tooth surface often is acid etched, so that a micro-mechanical interlock is achieved between the resin and the etched surface. Alternatively, as described in our copending U.S. patent application Ser. No. 235,166 (now U.S. Pat. No. 4,382,792), the disclosure of which is incorporated herein by reference, the surface of the tooth may be provided with a crystal growth to which the resin is bonded.
There is also a mechanical bond between the bracket base abutting the tooth and the resin. This mechanical bond represents the weak point of the assembly and brackets can become detached from the teeth by failure of the mechanical bond at the bracket/resin interface, if the shear strength thereof is exceeded. A variety of base designs have been proposed and utilized in an attempt to overcome this problem Some examples include a stainless steel base with perforations, a stainless steel mesh base, a foil mesh base with a thin piece of stainless steel welded to a mesh backing, a metal base with machined retentive undercuts, plastic brackets and bases constructed cf polycarbonate, and ceramic bases and brackets.
Although these bracket designs provide reasonably good bonding, there are a number of practical disadvantages associated with the designs. For example, perforated stainless steel bases may produce staining around the perforations at the adhesive level. In welded brackets, weld spots may obliterate the retentive areas in mesh bases, leading to decreased bond strength, and weld spots at the edges of the base may lead to leakage and corrosion. Plastic bases are difficult to bond to with current resin bonding systems while ceramic bases are bulky and may show brittle characteristics.